Spyware is application or software code that is written purposefully to steal the user's data like credit card details, social security numbers, passwords, etc. or to spy on the user's online surfing habits. Spyware is generally written so that it installs itself without the computer user's permission and in majority of the cases the person may not be even aware that his/her activities are being observed. Some spyware can also hijack web browsers, show unsolicited popups and download other malware, viruses or other malicious spyware. Spyware deletion can be sometimes difficult because they are often hidden from the user.

Before the word "spyware" meant computer hardware that was intended for espionage roles. However, after it was mentioned in a press release in the year 2000 by Gregor Freud, a security expert at Zone Labs, the term has taken on its present significance. Today, almost all the people who are familiar with computers and the Internet might be aware of viruses, spyware applications and trojan horses.

Spyware applications commonly install themselves using bugs in the web browser or when the user unknowingly downloads malicious or "tagged" files from CDs. The main difference between viruses and spyware is that dissimilar to virus programs, apyware applications do not replicate i.e. make exact copies of themselves and spread to other computers. Some trojans also download and install spyware or other malware without the computer user knowing anything about it.

Microsoft Windows is more insecure to spyware applications and other such malicious programs. Apple's MAC OS and other OSes like Linux and BSD are less vulnerable compared to Microsoft Windows. There is a lot of debating as to why there are few spyware applications targeting Linux and FreeBSD. Some that these OSes are design-wise less vulnerable and it is problematic to write spyware to function on these. Others claim that since the market share of these platforms is way lower when compared to Microsoft Windows, spyware authors do not think that it is worth the effort to program spyware specifically for these platforms.

If you use a Windows based computer, it is recommended to periodically apply updates to your OS to protect yourself from spyware and adware. Windows updates contain rectifications and patches to security vulnerabilities. It is also recommended to switch to another Web browser like Firefox or Opera in place of the default browser-Internet explorer. These web browsers are much more safe and can help in keeping your computer free from spyware.

You should also download and install a genuine virus scanner and keep it upto date. Many good anti-virus tools can disinfect spyware and adware. However, you can also install an anti-spyware software in addition to anti-virus for maximum benefit. A firewall software should also be installed. Many anti-virus packages or Internet security suites, as they are known, come with a firewall.